The Water Pollution Prevention Law was revised in June, 2001, and, in addition to the conventional regulation of the discharge of heavy metals and organochlorine compounds, the regulation of discharge of boron started. The standard for discharge of boron and its compounds is set below 10 mg per 1 L of water (below 10 ppm) in freshwater environments such as a river, and below 230 mg per 1 L of water (below 230 ppm) in seawater environments.
Conventionally, the following methods have been employed for treating boron-containing waste water: a method in which magnesium oxide in an amount 5 to 10 times or more (molar ratio) as large as that of boron is added to a boron-containing waste water, the pH is adjusted to 10 or higher with alkali metal hydroxide to form insoluble precipitate, and then the formed insoluble precipitate is removed; a method in which a large amount of aluminum salt is added, the pH is adjusted to 10 or higher with slaked lime to form insoluble precipitate, and the insoluble precipitate is removed similarly as in the above-mentioned method; a method in which zirconium oxide in an amount equivalent to that of boron is added to form insoluble precipitate, and the insoluble precipitate is removed; and a method in which a boron-selective ion exchange resin is brought into contact with a boron-containing waste water to remove boron by adsorption.
However, the methods in which an insoluble precipitate is formed using magnesium oxide or aluminum salt for removing boron each require a large amount of chemical agent which generate a large amount of precipitate, and thus are not economical. According to the method of removing boron using zirconium oxide, boron can be removed with a smaller amount of chemical agent and the amount of generated precipitate is smaller as compared with the above-mentioned methods, but the solid-liquid-separation properties of the generated sludge is low, and moreover, since zirconium is a rare metal, such a method is not economical. The method using a boron-selective ion exchange resin also requires a large amount of expensive boron-selective ion exchange resin, and thus is not economical.
Under such a technical background, methods of removing boron that are performed simply and with low cost and that have sufficient effects have been studied. For example, there has been reported a method of removing boron in a solution using a gel-like polyvinyl alcohol (JP 2002-186976 A). However, according to this method, boron is adsorbed on the surface of the gel-like polyvinyl alcohol for separating a boron-binding gel. Thus, although a large amount of chemical agent is used, the amount of boron that can be bound is small. In order to remove boron from an aqueous solution containing boron at high concentration and to reduce the boron concentration below 10 ppm by using this method, there arises a problem in that a large amount of chemical agent and a prolonged reaction time are required. Moreover, the shape of the gel needs to be adjusted at the gel formation phase in order that a large amount of boron is bonded. In addition, there is a problem in that the strength and manner of use of the gel need to be adjusted, and thus the above-mentioned method is far from being a simple method.